1. Field of Invention
This invention relates to an electromagnetic flow meter for converting the flow rate of a fluid to be measured to an electrical signal and for outputting a flow rate signal corresponding to the flow rate through detecting electrodes; and more particularly, to a power efficient flow meter which has increased signal to noise ratio (S/N) and reduced energy requirements for excitation and signal processing.
2. Description of the Related Art
Conventional electromagnetic flowmeters are known which intermittently supply an exciting current to an exciting coil to reduce the excitation energy.
One example is an electromagnetic flow meter disclosed in Japan Published Unexamined Application SN 54/115,163, wherein the flowmeter uses low frequency excitation in which the excitation period is shorter than the non-excitation period to reduce the mean power consumption. However, in this example, since signal sampling occurs after the differential noise, due to the increased excitation, disappears, the ON period (.tau.ON) of the excitation becomes long. Hence, the excitation cycle also becomes long when a predetermined repetition cycle is attempted. This results in the disadvantage that response is slow.
Another example is an electromagnetic flowmeter disclosed in Japan Published Unexamined Application SN 55/33,685, wherein a two-wire electromagnetic flowmeter uses power transmitted as a current signal from a DC power supply on the load side to the transmitter side through two transmission lines. In this flowmeter, the power supply supplies all of the power required at the transmitter side, and the detected flow rate signal is transmitted as a current signal to the load side through the transmission lines. In this example, power consumption is low, but, there is a problem in that the power for excitation is large and response is slow.
A further example is disclosed in Japan Published Unexamined Application SN 55/76,912, wherein an external signal is monitored, for example a signal representing the electrode potential, and excitation is made only when a variation exists in the electrode potential, thereby to reduce the overall excitation power. In this example, since the flowmeter carries out signal sampling when a steady-state value is obtained, the time taken for the steady-state value to be reached becomes long, which leads to the disadvantage that the power of excitation is large.
Another example is disclosed in Japan Published Unexamined Application SN 62/113,019, wherein positive and negative pulse-like exciting currents are supplied to an exciting coil, and signals are sampled during the time, including each excitation period and until the differential noise disappears, thereby to remove the differential noise, and a flow rate signal is obtained from the voltage difference between the positive and negative excitation levels by synchronous rectification. In this example, the flowmeter cannot uniquely determine the attenuation of the differential noise. Accordingly, if the sampling period is set to be sufficiently long, the noise, during this period, increases to thereby degrade the signal-to-noise ratio. On the other hand, if this period is set to be short, the effect of the differential noise appears. In addition, since a synchronous rectification is performed, signal processing in the low frequency region is necessary. Moreover, since the synchronous rectification by the sample values based on first and second reference pulses and the calculation of the difference between them are required, there is the problem that the hardware and software required to carryout the functions are complex.